fix Typo on OpenGLRenderer (libhwui) Properties

See: http://code.google.com/p/android/issues/detail?id=60165

In file

https://android.googlesource.com/platform/docs/source.android.com/+/master/src/devices/tuning.jd

Line 109 says property is called "texture_cache_flush_rate" however in the code it is called "texture_cache_flushrate". See that in file

https://android.googlesource.com/platform/frameworks/base/+/master/libs/hwui/Properties.h

Change-Id: I4b6aff888608e9d7c9de66421f68114081250b9b